ll-merge.c: refactor `read_merge_config()` to use `git_config_string()`

ll-merge.c: refactor `read_merge_config()` to use `git_config_string()`

There is one slight behavior change, previously "merge.default"
silently ignored a NULL value and didn't raise any error. But,
in the same function, all other values raise an error on a NULL
value. So to conform with other call sites in Git, a NULL value
for "merge.default" raises an error.

The the new config-set API is not very useful here, because much of
the function is dedicated to processing "merge.<name>.variable",
which the new API does not handle well.  If it were for variables
like, "merge.summary", "merge.tool", and "merge.verbosity", we could
use the new API.

Signed-off-by: Tanay Abhra <tanayabh@gmail.com>
Reviewed-by: Matthieu Moy <Matthieu.Moy@imag.fr>
Signed-off-by: Junio C Hamano <gitster@pobox.com>